Hi,
if you don’t have any special requirement, maybe you can grab a precompiled ROOT rather than compile it yourself: we have binary releases for ubuntu 18, that you just need to untar (and then you need to source $ROOTSYS/bin/thisroot.sh to add ROOT to your environment).
If you want to compile ROOT yourself you should use a newer version than 6.08 (6.08 is more than 5 years old and Ubuntu 18 is from this year, there might be mismatches in package versions).
The latest release is v6.14.04, but 6.16 is coming very soon.
Also, make sure to use cmake rather than ./configure; make, the latter build method has been deprecated after moving to C++ de-facto standard cmake build system.
Cheers,
Enrico